Conventionally, unitized case loads, such as multiple cartons of finished case goods stacked on top of each other and in side-by-side rows, and forming generally rectangular unitized loads of several thousand pounds, are conventionally borne by either wood pallets upon which the unitized case goods are stacked or slip sheets sized to the load, underlying the same and incorporating lips which project from beneath the case goods to one side thereof for facilitating case load transport.
Wooden pallets have been an important tool in handling of unitized case loads in the past century. Finished cased goods, such as canned foods and the like, are normally inventoried on wood pallets prior to their distribution. A typical wood pallet costs approximately $7.00, and while useful, they require replacement periodically. Further, such wood pallets are relatively heavy, weighing fifty pounds or so, and take considerable space.
Further, material handling systems involve the use of forklift trucks or the like having a pair of forks which readily project underneath and into these wood pallets to permit lifting of the wooden pallet and finished case goods for transport thereof. The more modern forklift trucks include vertical elevator systems as well as lateral extension and retraction of the forks borne by the elevator mechanism to facilitate loading, unloading, and stacking of the pallet borne loads.
As may be appreciated, not only do the wood pallets take up space when warehousing the case goods, but during transport, as for instance within tractor trailers, railroad boxcars or the like, the mass of the wooden pallets reduce the case goods content of the shipment while materially contributing to the overall weight of the shipment. Pallets significantly reduce the payload, thus increasing the per unit freight costs for the case goods.
In an attempt to substantially reduce the cost of material handling systems, thin rectangular slip sheets made of cardboard, solid fiber or the like, have been employed in material handling systems for handling the same unitized case goods loads that wood pallets handle. Slip sheets typically cost about one-tenth that of wood pallets. The average slip sheet weighs less than five pounds, compared to the fifty pounds for wooden pallets. The life of the slip sheets, however, is significantly less than that of the wooden pallets. Further, the slip sheets comprise nothing more than a rectangular piece of thin sheet material normally having a lip integral with the sheet along one edge or more, which projects outwardly of he load. The load covers the rectangular central portion of the slip sheet, while the projecting lip facilitates push-pull movement of the slip sheet and case goods. In that respect, to reduce labor hours in handling of the finished case goods, a forklift with a push-pull attachment is employed. The push-pull attachment is constituted by two components. One or more vertically reciprocating clamps are mounted above and are projectable down to and against the forks of the forklift truck, or an underlying rigid, inclined support surface. Typically, the vertical clamping mechanism is carried by a vertical wall known as load alignment member or face plate which is moved into contact with the front wall of the unitized case load, which load alignment member is projectable away from a narrow vertical pantagraph frame at the forklift truck body or chassis and outwardly beyond the underlying forks at the base of the chassis. Once the lip is clamped, the lip, flap, or tab acts as the fulcrum for the slip sheet, permitting the slip sheet to be pulled back onto and up the slope of the inclined forks until the slip sheet is fully supported by the forks (or their equivalent). If the forklift truck has its push-pull mechanism supported by an elevator, the elevator permits the forks, the push-pull unit and the load to be lifted vertically for multiple stacking of the unitized loads, one upon each other, separated by the slip sheet which is left under the newly stacked load and between the bottom of that unitized case load and the top of the immediately underlying unitized case load.
While such conventional slip sheet type material handling systems have substantially reduced the cost over material handling system utilizing wood pallets, they require forklift trucks capable of handling the full weight of the loads imposed on the slip sheets during push-pull operation. This, in turn, requires the slip sheets to be of relatively high strength, since intense tractive stresses exist in the slip sheets when the slip sheets are pulled across the underlying support surface with the case loads thereon. Further, large size, high load capacity motors are required to push, pull and lift the loads borne by the slip sheets.
In recent years, the applicant in conjunction with William D. Fletcher developed a low cost planar air pallet material handling system, as exemplified by U.S. Pat. No. 3,948,344 issuing Apr. 6, 1976. The system therein includes use of a modified forklift truck or like vehicle having projecting forwardly of the vehicle a pair of forks extending horizontally from a vertical plate, to the rear of which, is provided an open rectangular vertical frame functioning as a backing for the load supported on the forks. The load, in this case, is supported on an air pallet. In U.S. Pat. No. 3,948,344, the forklift forks penetrate plenum chambers, interior of the flexible thin sheet air pallet. The forks carry passages for delivering air under low pressure and at low CFM to selective plenum chambers defined by multiple flexible thin sheets bearing perforations in the load area and sandwiching an intermediate imperforate sheet having internal air dispersion means. The forks which penetrate air inlets for respective plenum chambers defined by a perforated sheet and the central imperforate sheet, produce selectively, as a result of air passage therethrough, an air bearing between the load and the air pallet or between the air pallet and the underlying support surface. As may be appreciated, by creating an air bearing between the air pallet and an underlying support surface, very little energy is required to slide the load off the underlying support surface due to the frictionless support of the same.
In other embodiments of the invention of U.S. Pat. No. 3,948,344, mechanical clamps are employed for clamping about an air pallet carrying a unitized case load constituted by as series of boxes or cartons stacked in side-by-side fashion. An edge of the air pallet extends outwardly from beneath the load and permits an air clamp to be coupled thereto. Pressurized air delivered through tubing via the air clamp and a passage therethrough permits flow of air under low pressure into the plenum chamber and thence out of the multiple perforations to form a low pressure, low CFR air bearing beneath the air pallet and between the air pallet and the underlying planar support surface. A very light, manual pulling or pushing force exerted by the operator carries the air pallet and its load to move effortlessly on a frictionless air bearing.
While the content of the low cost planar air pallet material handling system of U.S. Pat. No. 3,948,344 provides perhaps a third and seperate alternative to use of wood pallet type material handling systems and slip sheet material handling systems, no attempts have been made to integrate such air pallet concepts to the conventional slip sheet material handling sytems and to utilize the best features of each.
It is, therefore, a primary object of the present invention to provide an improved integrated air bearing slip sheet material handling system as a replacement for wood pallet or standard slip sheet type material handling system, and wherein the air bearing slip sheet permits forklift trucks and the like to operate with significantly size and mass reduced push-pull mechanisms, hydraulic, pneumatically or mechanically operated, along with the clamping mechanisms for clamping the slip sheet lip to the push-pull mechanism itself, materially reducing the overall cost of the forklift truck or the like.
It is a further object of the present invention to provide an improved air bearing slip sheet for use in such a material handling system in which the slip sheet is of simplified construction and in which air distribution throughout the plenum chamber of the air bearing slip sheet is assured, irrespective of the presence of the mechanical clamp necessary to facilitate push-pull movement of the slip sheet and load after air pressurization and creation of the air bearing between the slip sheet and the underlying relatively fixed support surface.